1. Field of the Invention
The present invention relates to a color-image pickup device which decomposes an optical image of an object with color filters, picks up the optical image with photoelectric conversion elements, and obtains a color-picture signal. In addition, the present invention also relates to an electronic color camera which contains the above color-image pickup device.
2. Description of the Related Art
The following document (1) discloses information related to the present invention.
(1) Japanese Unexamined Patent Publication No. 5-207350
In recent years, video cameras and electronic cameras have come into widespread use. Currently, the technology of advanced color-image pickup devices for use in the video cameras and electronic cameras is rapidly developing. Normally, CCD type or CMOS type image pickup elements are used in the color-image pickup devices. In order to obtain a color-picture signal, the image pickup elements are used in combination with on-chip type or field-sequential type color filters. In the on-chip type color filters, very small color filter elements are used in combination. In the field-sequential type color filters, color filter elements are sequentially arranged in a time-sharing manner. Further, the color filters may be RGB filters, YCyMg filters, or the like. The RGB filters are realized by R filters transmitting light in a red wavelength range, G filters transmitting light in a green wavelength range, and B filters transmitting light in a blue wavelength range. The YCyMg filters are realized by Y filters transmitting light in a yellow (red+green) wavelength range, Cy filters transmitting light in a cyan (green+blue) wavelength range, and Mg filters transmitting light in a magenta (red+blue) wavelength range.
Although it is often explained that the color filters are constituted by a plurality of optical filters each transmitting light in a predetermined wavelength range as indicated above, the color filters actually have a different structure. For example, since the image pickup devices such as CCDs have substantial sensitivity to infrared rays, it is necessary to provide an infrared-cutoff filter which cuts off the infrared rays.
Hereinbelow, the detailed structure of an example of an actual color filter will be described. In this example, the color filter is a primary-color filter. FIG. 12 shows transmission wavelength ranges of four optical filters (a B filter, a G filter, an R filter, and an infrared-cutoff filter) used in the primary-color filter. As shown in FIG. 12, the B filter transmits light in the wavelength range from about 400 to 450 nm, and determines a blue wavelength range, the G filter transmits light in the wavelength range from about 500 to 550 nm, and determines a green wavelength range, the R filter transmits light having wavelengths equal to or longer than 600 nm, and determines the lower limit of the red wavelength range, and the infrared-cutoff filter determines the higher limit of the red wavelength range. Since neither the B filter nor the G filter can cut off the infrared rays, in many color filters, the infrared-cutoff filter is superimposed on each of the R, G, and B filters, and normally a discrete infrared-cutoff filter such as a colored glass plate is inserted into the optical system.
Further, with the recent development in the miniaturization of color-image pickup devices, there is great demand for the miniaturization of optical elements. Therefore, currently, the miniaturization of image pickup lens systems is being pursued. In addition, in some proposed devices, as disclosed in the aforementioned document (1), a dielectric multilayer film for infrared blocking is formed by vapor deposition on an optical element such as a lens or cover glass, instead of the insertion of the discrete infrared-cutoff filter.
Increase in the angular aperture on the image side (i.e., increase in the angle between the optical axis and the principal ray corresponding to the maximum image height) is effective to miniaturize the image pickup lens systems. However, when the angular aperture on the image side is great, the light incident angle at each photoelectric conversion element in each image pickup device increases as the distance from the center of the light-reception area of the image pickup device to the photoelectric conversion element increases. On the other hand, as illustrated in FIG. 13, the transmission wavelength range of the dielectric multilayer film for infrared blocking has a dependence on the incident angle, and the red wavelength range moves to the shorter-wavelength side as the incident angle increases. Therefore, the upper-wavelength side of the red wavelength range is cut off, and the width of the red wavelength range is reduced.
Thus, the ratio of the signal intensity in the red wavelength range to the signal intensity in the green or blue wavelength range at each photoelectric conversion element in each image pickup device decreases as the distance from the center of the light-reception area of the image pickup device to the photoelectric conversion element increases. Therefore, colors represented by the color signal obtained from the above image pickup device are different from actual colors, i.e., color shading occurs.